Popular music is like sheep chasing after the shepard with the most fashionable grass.
Patterns in music tend to follow trends that initiate from an underground sub-culture. It always stems from some "vague" source, becomes more and more familiar to larger and larger groups of people and eventually becomes the new thing. Once a big "hit" is recognized commercially, all the major labels (shepards) run around looking for a similar artist, and put them into play. Soon the airwaves are saturated with the same sound (Green Day >> Blink 182 >> MXPX >> New Found Glory >> Sum 41; or Backstreet Boys >> NSync >> 98 Degrees; Nirvana >> Pearl Jam >> Sound Garden >> Bush >> Stone Temple Pilots; etc.). People get bored of hearing the same thing and begin to look for new sounds. Meanwhile, the Majors are looking to the Underground Music scene to find what's the next hot thing (Emo? Ska? Hardcore? Oi? Math Rock?), and sign some to test the waters. If they hit, they become the new genre definer, and then all the labels rush in to fill the void of the sole newcomer.
Good bands that last, usually buck the trends and constantly redefine themselves, not necessiarily following new trends, but trying to supercede the mold (Maddonna, U2, REM). Bands and artists that can't, fall into the category (Hair bands, ska, heavy metal, raprock), and die when the genre dies. Where are they now? After a genre dies, it's forgotten for a while, and potentially cycles back in time when it's new again (punk, ska, rap-metal) either as the same forms or in new hybrid forms.
This whole process is call co-opting. This is the process of digging into the subcultures and making them mainstream, often destroying the context and true meaning and symbolism of the original subculture. Jocks moshing is not how it was meant to be.